Beware superstitious friends: It's Friday the 13th again! Here are some fun facts about the day you might not know:

Fear of the day is likely rooted in Christianity.

Jesus was crucified on a Friday and ever since the day has been associated with "general ill omen," Michael Bailey, a history professor at Iowa State University who specializes in the origins of superstitions, told USA TODAY Network. Weddings in the Middle Ages, for instance, were not held on Fridays and it was not a day someone would start a journey, Bailey said.

Thirteen guests are believed to have attended the Last Supper, the night before Jesus was killed, according to Stuart Vyse, a psychology professor at Connecticut College. And Judas Iscariot, the disciple who betrayed Jesus, is considered to have been the 13th guest, Vyse said.

The superstition's origins are mysterious.

It's unclear when Friday and number 13 became linked in the way we think of them today, according to Vyse and Bailey. There are no mentions of Friday the 13th before the 19th century.